Fluid mixtures can be separated by selective diffusion through membranes under concentration or pressure gradients by utilizing differences in transport and thermodynamic partition or equilibrium properties of the mixture components in the membrane materials. One widely-used type of membrane comprises a non-porous polymer in which the mixture components selectively dissolve and selectively permeate or diffuse in the soluble state through the polymer to yield a permeate product enriched in the selectively diffusing components and a non-permeate or reject product enriched in the remaining components. A second type of membrane comprises a porous solid in which the mixture components selectively diffuse or permeate in the fluid state through the pores to yield a permeate product enriched in the selectively diffusing components and a non-permeate or reject product enriched in the remaining components.
There are four mechanisms by which fluid mixtures, in particular gas mixtures, can be separated by a solid porous membrane. The first of these is diffusion in the gas phase through pores having diameters approaching the mean free path dimensions of the molecules in the gas mixture, which is often termed Knudsen flow or Knudsen diffusion. These pores are small enough, however, to preclude bulk gas flow by molecular diffusion. In Knudsen flow, the permeation rate of each component is inversely proportional to the square root of its molecular weight. The phenomenon of gas diffusion and separation by Knudsen flow through porous solids is well known, and is described in standard textbooks such as "Mass Transfer in Heterogeneous Catalysis", by C. N. Satterfield, MIT Press, 1969.
A second type of mechanism for the separation of gas mixtures by porous solids occurs when the diameters of the pores are larger than the largest molecular diameter of the components in the gas mixture and range up to about 40-100 Angstroms in diameter. At the appropriate temperature and pressure conditions, certain components in the gas mixture will condense within the pores by capillary or Kelvin condensation and flow through the pores as a condensed phase under a capillary pressure gradient across the membrane. Condensed molecules within the pores hinder or eliminate the diffusion of non-condensing molecules, and a selective separation between components in the gas mixture is thus accomplished.
A third type of separation mechanism occurs when the pore diameters of the membrane are larger than the largest molecular diameter of the components in the gas mixture and typically smaller than about 2 to 5 times this diameter, and thus are smaller than pores in which Knudsen diffusion dominates. These pores have typical diameters of about 3 to 20 Angstroms and are termed micropores by the classification definition of the International Union of Pure and Applied Chemistry (I.U.P.A.C.). In the present disclosure, the term "pores" will be used to denote pores of any size, including micropores. When a gas mixture is contacted with such a porous membrane, the separation mechanism defined as selective surface flow or selective surface diffusion can occur under a pressure gradient across the membrane. This mechanism is characterized by the selective adsorption of certain mixture component molecules within the pores and the surface flow of these molecules in the adsorbed phase through the pores. Furthermore, the adsorbed phase hinders the gas-phase diffusion of non-adsorbed or weakly adsorbed component molecules through the pores, and an enhanced selective separation between components in the gas mixture is thus accomplished.
The fourth mechanism by which gas mixtures are separated by a solid porous membrane material is that of molecular sieving in which essentially all of the pores are larger than certain component molecules and smaller than other component molecules in the mixture. Larger molecules cannot enter these pores or are substantially excluded, while smaller molecules can enter and diffuse through the pores, and a selective separation based upon exclusion by molecular size is thus accomplished.
Since porous solids contain a distribution of pore sizes, more than one of these mechanisms can occur simultaneously depending upon the actual pore size distribution and sizes of component molecules in the gas mixture, as well as the pressure and temperature. However, a single mechanism usually dominates and the resulting mixture separation is essentially accomplished by means of that dominant mechanism.
U.S. Pat. No. 5,104,425 discloses a composite semipermeable membrane comprising porous adsorptive material supported by a porous substrate, a series of methods for making the membrane, and a process for the separation of multicomponent fluid mixtures utilizing the third of the four mechanisms described above. The separation is accomplished by bringing the fluid mixture into contact with a first surface of the membrane, wherein significant portions of certain components are selectively adsorbed within the pores in the adsorptive material and permeate through the pores by surface flow in an adsorbed phase to produce a permeate product enriched in these components. The remaining fluid mixture is withdrawn from contact with the membrane to yield a nonpermeate fluid product enriched in the remaining components.
The membrane is made by coating a surface of a porous substrate with a layer of a precursor material, heating the resulting coated porous substrate in an inert atmosphere to a temperature sufficient to convert the precursor material into a layer of microporous adsorptive material, and cooling the resulting composite membrane to ambient temperature. Precursor materials can include polymeric materials which are carbonized by heating in an inert atmosphere to form a layer of microporous carbon on the surface of the substrate or Inorganic materials which are dried and crystallized to form a layer of porous inorganic adsorbent material on the surface of the substrate. The microporous adsorptive material forms a layer up to about 20 microns thick.
A method is also disclosed for making a densified composite semipermeable membrane by introducing a precursor into the pores of a porous substrate, heating the porous substrate containing the precursor under conditions sufficient to convert the precursor to porous adsorptive material within the pores, and cooling the resulting composite membrane to ambient temperature. The porous adsorptive material can be activated carbon formed by the carbonization of polymeric materials or by the deposition of carbon by vapor phase cracking of gaseous hydrocarbons followed by activation in an oxidizing atmosphere.
The preferred method of making these membranes utilizes a polymeric precursor to produce carbon as the porous adsorptive material which separates the gas mixtures of Interest. These membranes are useful for recovering hydrogen from mixtures with light hydrocarbons such as methane, ethane, propane, and butane to yield a permeate enriched in the hydrocarbons and a hydrogen-rich product essentially at the feed pressure.
In industrially important gas separations such as the recovery of hydrogen, improved separation efficiency and process economy are desirable when utilizing adsorptive membranes. Such improvements can be achieved by increasing the permeability and selectivity of these membranes for hydrocarbons by methods of the present invention as described in the following specification and claims.